Is hay the only substrate you have in there? I know humidity isn't a huge concern with Russians, but hay can be kind of drying. If you just had some in a hiding area, though, I'd imagine that would be okay.
We'd love to see some pictures of your tortoise's burrowing antics
Russians don't need a high humidity. If he is an adult, around 30-40% is fine. However, I would not use the hay in a large area of the enclosure. A hide area or a little in a corner or so is fine.
One of my enclosure I used hay (Timothy hay) before. Not good. Very hard for baby tortoise to walk around because the front leg of the tortoise may make some hay to stand-up when they stepped on and the hind leg will get caught like a spear poke the little thing.
And mold....................................................................